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Electronics basics: voltage, current, resistance and Ohm's law

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tagsglossar, elektronik, grundlagen

3 min read

Electronics basics: voltage, current, resistance and Ohm’s law

In short: Voltage (V) drives current (A) through a resistance (Ω); Ohm’s law U = R · I and power P = U · I explain most circuits.

In more detail: Whoever tinkers with hardware, repairs it or chooses power supplies needs three quantities. A water picture helps: voltage = pressure, current = amount of water per second, resistance = narrowness of the pipe.

In Depth

The quantities

  • Voltage U in volts (V): potential difference; AA battery 1.5 V, USB 5 V (more with PD), wall socket 230 V.
  • Current I in amperes (A): flowing charge; mA = thousandth.
  • Resistance R in ohms (Ω): inhibits current.
  • Power P in watts (W): P = U · I; energy in watt-hours (Wh) or kWh.
  • Direct current (DC) (battery, USB) vs. alternating current (AC) (wall socket 50 Hz).

Ohm’s law

U = R · I → I = U / R, R = U / I. LED example: supply 5 V, red LED (≈ 2 V voltage drop, 10 mA wanted): series resistor R = (5 V − 2 V) / 0.010 A = 300 Ω → take 330 Ω. Power in the resistor: P = U·I = 3 V · 0.01 A = 0.03 W (a 1/4 W resistor is enough). Without a series resistor the LED burns out; it is not a light bulb but current-controlled.

Series and parallel circuits

  • Series: same current, voltages add, resistances add (R = R1 + R2).
  • Parallel: same voltage, currents add, reciprocals add (1/R = 1/R1 + 1/R2; two equal → half the value).
  • Voltage divider: Uout = Uin · R2/(R1+R2), used for sensors (photoresistor, potentiometer) and level adaptation.
  • Kirchhoff: junction rule (sum of currents = 0) and loop rule (sum of voltages = 0).

Safety and everyday life

  • Mains voltage (230 V) is life-threatening; circuits and experiments only with extra-low voltage (battery, USB, power supply with protective extra-low voltage). Never work on mains-powered devices without expertise (power supplies, CRT TVs, capacitors store charge!).
  • A short circuit causes high currents, heat, fire risk; batteries (LiPo) especially carefully.
  • Power supply example: 12 V and 2 A → 24 W; a device needing 12 V / 1.5 A works on it; polarity and voltage must match, current (A) may be higher, not lower.
  • Electricity cost: 100 W · 10 h = 1 kWh ≈ 30 to 40 ct.
  • Practise with simulators (Falstad, Tinkercad Circuits, LTspice).

See also: Power supplies, Arduino and microcontrollers

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